Abstract: A Session Initiation Protocol Application Server for use within an IP Multimedia Subsystem. The Application Server comprises a receiving unit for receiving a Session Initiation Protocol message from a Serving Call Session Control Function, the Serving Call Session Control Function serving an IP Multimedia Subsystem user and the message containing within a message header an explicit identification of said user. A processing unit determines an action to be applied to said message and includes within a header of the message a role value defining a role of said user in respect of the action. A transmitter unit returns the message including the role value to said Serving Call Session Control Function.